Encapsulants used to protect solar cells from the environment must possess several characteristics. First, the encapsulant must be easily deformable, prior to being cured, so it can conform to the shape of the solar cell for proper sealing without breaking the relatively fragile solar cell. Also, the cured formulation of the encapsulant must be a thermoset compound, i.e. a compound which is resistant to flow when exposed to heat. Because a solar cell is exposed to heat during its normal operation, a compound which readily flows when exposed to heat, i.e. a thermoplastic compound, would not be suitable for use as an encapsulant. A thermoplastic compound would likely flow off of the solar cell, thereby causing the solar cell to become exposed during its normal operation. Additionally, the encapsulant must be adhesive to adhere to the cell and other components during operation. Finally, the encapsulant must be optically transparent.
Ethylene-vinyl acetate (EVA) co-polymers are among the most widely used encapsulants for the manufacture of solar cells, or solar modules. However, field experience shows that a gradual discoloration develops within the encapsulant. This discoloration lowers the power output of the module by approximately 5%. This discoloration effect has been primarily noticed in hot environments; however, EVA encapsulants in other environments also tend to eventually show discoloration. This discoloration may be caused by exposure to heat, i.e. thermal discoloration, and/or by exposure to light, such as ultraviolet light. The discoloration appears as a yellowing of the EVA encapsulant, which is an absorption in the blue end of the spectrum, and decreases the power conversion in this range.
In the past, some additives have been used to attempt to minimize encapsulant discoloration. One such additive is the compound Bis-(Tetramethyl piperidinyl sebacate), produced by Ciba-Geigy Corporation of Hawthorne, N.Y. and sold under the trademark "TINUVIN 770". However, a need still exists to further minimize thermal discoloration.